Sleep hypoxemia is frequent in patients with chronic obstructive lung disease (COLD). Both obstructive and central sleep apnea cause brief episodes of hypoxemia in such patients but prolonged severe hypoxemia is often not associated with any type of apnea. By measuring breathing patterns, lung volumes and continuous oxygen saturation and CO2 tension, this study will evaluate the contribution of alveolar hypoventilation, ventilation/perfusion mismatch and apnea to the occurrence of sleep hypoxemia in patients with COLD. Oxygen will also be administered to determine the benefits of such therapy in apneic hypoxemia versus nonapneic hypoxemia. The effect of oxygen on length of apnea, level of CO2 tension, and improvement in arterial oxygen saturation will be measured. Evidence will be sought relating sleep apnea and desaturation to specific types of COLD; i.e. pure chronic bronchitis versus pure emphysema. Another objective of this study is to detect the frequency and severity of periodic breathing and desaturation during sleep in normal people. Preliminary data suggest that sleep apnea and arterial desaturation are not uncommon in subjects without lung disease. At least 50 normal people of varying age, sex, and weight will be monitored to answer this question. In all studies sleep will be verified and staged electroencephalographically and electroculographically.